1. Field of the Invention
The present invention relates to a self-sealing tyre for vehicle wheels.
More particularly, the present invention relates to a self-sealing tubeless tyre for vehicle wheels and to a process for manufacturing said tyre.
2. Description of the Related Art
In the industrial field of tyres for vehicle wheels, various attempts have been made to produce self-sealing tyres, in other words tyres which may retard or prevent the loss of air and consequent deflation of the tyres themselves following a puncture caused e.g. by a sharp object (a nail, for example).
To achieve the above goal, self-sealing tyres are known which are provided with at least one layer comprising a polymeric material which may adhere to the object causing the puncture and may also flow into the puncture site when said object is removed, thus sealing the puncture and preventing the outflow of air from the tyre.
For example, U.S. Pat. No. 3,981,342 describes a self-sealing tubeless tyre provided with a layer including a composition comprising a mixture of a low molecular weight liquid elastomer and a high molecular weight solid elastomer, and a quantity of a crosslinking agent sufficient to give a partial crosslinking of said mixture, said liquid elastomer being present in a greater quantity than said solid elastomer. Examples of low molecular weight liquid elastomers which may be used according to the invention are: liquid cis-polyisoprene (for example, heat-depolymerized natural rubber, or cis-polyisoprene polymerized to low molecular weight); liquid polybutadiene; liquid polybutene; liquid EPDM; liquid butyl rubber. Examples of high molecular weight elastomers which may be used according to the invention are highly unsaturated rubbers such as those based on conjugated diolefins, whether homopolymers as in polyisoprene (particularly natural or synthetic cis-polyisoprene); polybutadiene (including polybutadiene with a high cis content); polychloroprene (neoprene); or copolymers as exemplified by those having a major proportion of such conjugated dienes as butadiene with a minor proportion of such monoethylenically unsaturated copolymerizable monomers as styrene or acrylonitrile. Alternatively, it is possible to use elastomers with a low degree of unsaturation such as butyl rubbers (isoolefin copolymers, for example isobutylene, with small quantities of dienes, for example isoprene), or EPDM-type rubbers (copolymers of at least two different monoolefins such as ethylene and propylene with a small quantity of a non-conjugated diene such as dicyclopentadiene, 1,4-hexadiene, 5-ethylidene-2-norbornene). Finally, it is also possible to use saturated elastomers such as EPM or ethylene-vinyl acetate copolymers. Examples of crosslinking agents which may be used according to the invention are: sulhur or sulfur donors; quinones; organic peroxides or hydroperoxides; polyisocyanates; and tetrahydrocarbyl titanate esters. Said layer would be able to impart self-sealing properties to the tyre.
Patent application EP 127 998 describes a tubeless tyre wherein the inner liner is a laminar structure formed by two outer layers comprising a halogenated butyl rubber and an inner layer comprising a butyl rubber and an organic peroxide. The inner layer comprising butyl rubber preferably contains, per 100 part by weight of butyl rubber, from 40 to about 100 part by weight of one or more type of carbon black, from about 5 to about 40 part by weight as plasticizers of one or more fluid polyisobutylenes or copolymers containing polyisobutylene, from about 5 to about 30 parts by weight of one or more hydrocarbon oils, especially paraffinic hydrocarbon oil, from about 2 to 20 parts by weight of one or more tackifiers such as hydrocarbon or hydrogenated wood rosin tackifiers, from about 0.5 to about 3 parts by weight of other processing aids such as stearic acid and the like, and a peroxidic compound which may be selected from organic peroxides and inorganic peroxides. The amount of such organic peroxides, defined on the basis of pure material, is from about 2 to about 4 parts by weight per 100 part by weight of butyl rubber. Suitable inorganic peroxides include zinc peroxide and the amount of zinc peroxide which may be used is from about 3 to about 10 parts by weight per 100 parts by weight of butyl polymer. After vulcanization, the butyl rubber of said inner layer would be degraded, because of the presence of the peroxidic compound, thus producing a polymeric material having a Mooney viscosity ML(1+4) at 100° C. in the range from 2 to 12. Said polymeric material would be able to impart self-sealing properties to the tyre.
U.S. Pat. No. 4,895,610 describes a self-sealing tyre provided with a layer comprising a butyl rubber and at least one peroxide vulcanizing agent. Preferably, said layer comprises: 100 parts of butyl rubber; about 10 to 40 parts of carbon black; about 5 to 35 parts of polyisobutylene; about 5 to 35 parts of an oil extender; about 0 to 1 parts of sulfur; and about 1 to 8 parts of a peroxide vulcanizing agent. During the vulcanization of the tyre, the butyl rubber, in the presence of the peroxide vulcanizing agent, would be capable of being partially cross-linked thus providing dimensional stability to said layer, and also of being partially degraded thus providing a low-viscosity tacky material capable of imparting self-sealing properties to said tyre.
U.S. Pat. No. 4,913,209 describes a self-sealing tubeless pneumatic tyre constructed upon a laminate of elastomers comprising an air-impervious liner or elastomeric strip (“inner liner”) coated with a thin layer of bare (that is, not covered and not constrained by a sheet of flexible material) elastomeric puncture-sealing material (“sealant”). The sealing material consists essentially of (i) a blend of a major amount by weight of high molecular weight elastomer, and a minor amount by weight of low molecular weight elastomer, based on the combined weight of the high and low molecular weight elastomers; (ii) at least as much processing aid, whether homogenizer or tackifier, as there is high molecular weight elastomer; and (iii) a curative in an amount sufficient to provide said sealant with a peak room temperature (75° F.) Mooney viscosity (MLP/rt) in the range of from above 70 to about 110 and, when cured, with a peak Mooney viscosity at 150° F. (MLP/150) below 30, preferably in the range of from above about 15 but less than 30. Examples of high molecular weight elastomers which may be used according to the invention are highly unsaturated rubbers such as, those based on conjugated diolefin, whether homopolymers as in polyisoprene (particularly natural or synthetic cis-polyisoprene); polybutadiene (including polybutadiene of high cis content); polychloroprene (neoprene); or copolymers as exemplified by those having a major proportion of such conjugated dienes such as butadiene with a minor proportion of such monoethylenically unsaturated copolymerizable monomers as styrene or acrylonitrile. Alternatively, elastomers of low unsaturation may be used, notably butyl rubbers (copolymers of such isoolefin as isobutylene with small amounts of conjugated dienes such as isoprene), or EPDM-type rubbers (copolymers of at least two different monoolefins such as ethylene and propylene with a small amount of a non-conjugated diene such as dicyclopentadiene, 1,4-hexadiene, 5-ethylidene-2-norbornene). Even saturated elastomers such as EPM or ethylene-vinyl acetate copolymers may be employed, using the proper cure system. Examples of low molecular weight elastomers which may be used according to the invention are liquid cis-polyisoprene (for example, heat-depolymerized natural rubber, or cis-polyisoprene polimerized to low molecular weight); liquid polybutadiene; liquid polybutene; liquid EPDM; liquid butyl rubber. Examples of homogenizing agents (homogenizers) which may be used in the invention are low molecular weight polymeric resin blends. Examples of tackifiers which may be used in the invention are low molecular weight material such as rosin ester; aliphatic petroleum hydrocarbon resins; polyterpene resins; resins made from styrene and related monomers; resins made from dicyclopentadiene; and resins obtained from the reaction of a mineral oil purification residue with formaldehyde and with nitric acid catalyst. Examples of crosslinking agents which may be used according to the invention are: sulfur or sulfur donors; quinones; organic peroxides or hydroperoxides; polyisocyanates; and tetrahydrocarbyl titanate esters. Said layer would be able to impart self-sealing properties to the tyre.
Nevertheless, the solutions proposed in the above-mentioned prior art show some drawbacks.
For example, in the case of degradation, the self-sealing layer has to be laminated between at least two other undergraded layers in order to be capable of retaining its structural integrity both during the vulcanization operation wherein high pressure are applied to the tyre which would displace the degraded rubber layer from its desired location if not laminated and during the use of the tyre. The problem is complicated by the extreme centrifugal force to which the sealing composition is subjected as the tyre rotates at high speed since said centrifugal force tends to cause the composition to flow into the central crown area leaving the areas near the shoulder unprotected.
In the case of crosslinking it is necessary to pay particulary attention to the degree of crosslinking of the final sealing composition. If the degree of crosslinking is too high the sealing composition does not possess a necessary flowability which allows the same to flow into the puncture hole, while if the degree of crosslinking is too low, the same drawbacks above-mentioned in the case of degraded sealing composition arise.